Multiple lines of evidence suggest that the propensity to develop glomerulosclerosis in man is under genetic control. The Os mutation induces a congenital 50% reduction in nephron number and both hypertrophy and increased labeling index in glomeruli. This mutation induced glomerulosclerosis (GS) in a susceptible strain (ROP mice), but not in a GS resistant (C57 mice) strain (J Clin Invest 1996, 97:1-8). The purposes of this study were to determine whether nephron reduction after birth induced GS in susceptible mice, and whether further reduction in nephron mass (75%) would induce GS in resistant strains. Adult ROP +/+, ROP Os/+, C57 +/+ and C57 Os/+ mice underwent unilateral nephrectomy (NX). Glomerular volume increased two weeks after nephrectomy in ROP +/+, C57 +/+ and ROP Os/+ but not in C57 Os/+ mice. Glomerular labeling index increased in ROP +/+ NX and C57 +/+ NX mice. Whereas C57 +/+ NX and C57 Os/+ NX mice showed no glomerular changes, ROP +/+ NX mice exhibited mild lesions and ROP Os/+ NX had severe GS. Glomerular 1IV collagen mRNA was higher in ROP +/+ NX than in C57 +/+ NX. 1IV collagen, laminin 1, and tenascin glomerular mRNAs increased in ROP/Os but not C57 Os/+ mice 2 months after nephrectomy. Thus, the response to adult nephron reduction depended on the mouse strain, since nephrectomy induced lesions only in ROP mice. The lesions in ROP +/+ NX were less severe than in sham operated ROP Os/+ mice, suggesting that the time at which nephron reduction occurs was critical in determining the amplitude of the sclerotic response, even in a susceptible strain. In order to further determine the nature of the genes implicated in glomerulosclerosis we identified a mouse strain that did not develop glomerulosclerosis (C3H), and crossed it with a sclerosis-prone mouse (ROP) strain carrying the Os mutation. The mutation was used to accelerate, or make more obvious, the tendency to develop sclerosis. A backcross of the F1 animals to the ROP background revealed that the resulting F2 generation mice had varying degrees of glomerulosclerosis at the age of 3 months. Thus, we could demonstrate that there was a clear-cut inheritance of the susceptibility to develop glomerulosclerosis in mice. The next steps in this investigation will be to attempt the localization and characterization of certain loci which are peculiar to those mice with the most severe lesions. Since the Os gene is of interest in renal development, we tried to localize this mutation using a PCR based mapping approach using forty SSLP markers in the center of chromosome 8. Based on the number of generations the animals have been backcrossed, since the mutation was induced in 1952, we expected to find a very small region. Surprisingly, our mapping data revealed a linkage dysequilibrium involving a region of 10 cM in length. By subsequent G-banding analysis and fluorescence in situ hybridization, we found that neither a large inversion nor a large deletion seems to be present in this part of the chromosome. We now have to consider that the defect is due to one or more smaller chromosomal abnormalities. Since lipid abnormalities are thought to be a potential source of renal injury, we characterized the renal phenotype of mice with induced lipid abnormalities. We chose to study mice transgenic for LCAT and Apo E knockout mice. On a sclerosis-resistant background we found that LCAT mice had lipid inclusions in glomerular endothelial and mesangial cells, but that there was very little sclerosis. Similar findings were present in the Apo E ko mice. We are now crossing these abnormalities onto a sclerosis-prone mouse background.